Discharge lamp

ABSTRACT

The invention relates to a discharge lamp ( 1 ) having an outer bulb ( 2 ) that is provided with a lamp cap ( 3 ) at one end. The outer bulb encloses a discharge vessel ( 4 ) comprising electrodes, a first current conductor ( 5 ) and a second current conductor ( 6 ) arranged at some distance from the first current conductor, which current conductors establish an electric connection between the electrodes and the lamp cap. The shortest distance between the electrodes forms a discharge axis ( 40 ) of the discharge vessel. Part of the second current conductor ( 6 ) is placed substantially sidelong to the discharge axis. According to the invention, the second current conductor ( 6 ) comprises successive parts ( 7,8,9 ) arranged sidelong to the discharge axis and at mutually different distances from said axis.

The invention relates to a discharge lamp comprising an outer bulb,which outer bulb is provided with a lamp cap at one end, said outer bulbaccommodating a discharge vessel provided with electrodes, and a firstpole and a second pole at some distance from the first pole, which polesestablish an electric connection between the lamp cap and theelectrodes, at least a part of the second pole being mainly laterallypositioned with respect to a discharge axis, and said discharge axisforming the shortest connection between the electrodes.

The discharge lamp mentioned in the opening paragraph has been known formany years from the prior art. An important drawback of the knowndischarge lamp resides in that a discharge channel present between theelectrodes in the discharge vessel does not always extend in a straightline. When the discharge lamp is in operation, the discharge channel maybe curved in shape, for example when the discharge lamp is operated in avertical position. Said curved shape of the discharge channel can beattributed to the fact that the second pole which is laterallypositioned with respect to the discharge vessel generates a tangentialmagnetic field during operation, causing a Lorentz force to be exertedon the charged particles forming the discharge channel. A drawback ofthe curved shape of the discharge channel resides in that it leads to anon-uniformly distributed thermal load on different parts of thedischarge vessel, as a result of which the temperatures of differentparts of the discharge vessel may differ substantially. The temperaturegradient thus developed may lead to thermomechanical stress in parts ofthe discharge vessel, particularly in discharge vessels manufacturedfrom a ceramic material. This physical effect may subsequently lead to apremature end of the service life of the lamp. The above negative effectis important, in particular, in discharge lamps that are arranged so asto be vertically positioned during operation, since to compensate acurved discharge channel use cannot be made of other compensatingeffects, such as a convective flow generated in the discharge vesselduring the discharge.

French patent specification FR 779256 describes a discharge lamp of thetype mentioned in the opening paragraph, wherein the second pole that islaterally positioned with respect to the discharge vessel is bilaterallypositioned with respect to the discharge vessel. The second polebifurcates into two diametrically opposite segments which are laterallyarranged with respect to the discharge vessel, which segments,consequently, generate a substantially equally large, yet oppositelydirected magnetic field at the location of the discharge channel.Consequently, the generated magnetic fields will substantiallycompensate each other in the discharge vessel. As the resultant magneticfield is thus minimized in the discharge vessel, no Lorentz force, oronly a very small Lorentz force, will act on the charged particles, as aresult of which the discharge channel is substantially rectilinearlypositioned between the two electrodes in the discharge vessel. As aresult, a substantial temperature gradient will normally be absent. Thelamp is arranged to be, in particular, vertically oriented duringoperation. The device described in this publication, however, has a fewdrawbacks. A first drawback of the device described in said publicationresides in that the second pole requires a double construction and henceis complex. This leads, inter alia, to the necessity of an additionalnumber of welding points. Such a complex construction involvescomparatively high manufacturing costs, while the manufacturing processis usually time consuming. In addition, as a result of said complexconstruction the risk of rejects during the production process isincreased. A second drawback resides in that the construction of thedevice described is very critical. If the intended ratio of the electricresistors of the individual segments of the dual construction of thesecond pole is not accurately realized, for example as a result ofimperfections in the welded joint, the current intensities through theindividual segments of the second pole will not have the desiredsubstantially equal value and, consequently, the intended compensatingeffect of the magnetic fields generated by the segments will not beachieved. The construction of the second pole is extremely critical,partly due to the reasons stated hereinabove, which is disadvantageous.

It is an object of the invention to provide a discharge lamp wherein theabove-mentioned drawbacks are obviated.

To achieve this object, the invention provides a discharge lamp of thetype mentioned in the opening paragraph, which is characterized in thatthe second pole is positioned unilaterally with respect to the dischargevessel, said second pole being shaped such that a magnetic field at thelocation of the discharge vessel is minimized. Since the resultantmagnetic field in the discharge vessel is minimized by the shape of thesecond pole, curvature of the discharge channel in the discharge vesseloccurs hardly or not at all. Consequently, the discharge channel will besubstantially rectilinear. The design of the second pole is simple anddoes not require a complex and extensive manufacturing process, i.e. forexample processing steps such as welding and soldering. The second pole,unlike the second pole in accordance with the prior art, can bemanufactured in a single processing step. This advantage is desirable inparticular in the case of lamps whose discharge vessel is mainlyvertically oriented during operation, because in said position of thedischarge vessel other compensating effects are absent.

The second pole is preferably provided with different, successive partswhich are laterally positioned with respect to the discharge axis in thedischarge vessel, which parts are spaced apart. The orientation of theparts with respect to each other is such that the resultant of themagnetic fields generated by the parts is only very small at thelocation of the discharge vessel. Preferably, the magnetic fieldsgenerated by the parts of the second pole extend in at least twoopposite directions. This can be achieved, for example, by bending thesecond pole in a number of locations, thereby causing the magneticfields generated by the individual parts to bend in the same direction.Thus, when the second pole is bent through 180°, a reversal of themagnetic field takes place.

In a preferred embodiment, the distance at which at least one part ofthe second pole is situated from the discharge vessel differs from thedistance at which the other parts of the second pole are situated fromsaid discharge vessel. This enables magnetic fields to be compensated ina simple way as, in general, it applies that the size of the magneticfield generated by a part at the location of the discharge vessel isinversely proportional to the distance between said part and thedischarge vessel. The parts of the second pole are preferably positionedin such a manner with respect to each other that the following applies${{\sum\limits_{i = 1}^{N}\quad\frac{{ni} \cdot I}{di}} \approx 0},$with N≧2 where:

-   ni=the direction of the magnetic field generated,-   N=the number of parts of the second pole that are laterally arranged    with respect to the discharge axis of the discharge vessel,-   I=the intensity of the current flowing through the discharge channel    in the operating state, and-   di=the distance between a certain part of the second pole and the    discharge axis of the discharge vessel.

As mentioned above, the direction of the magnetic field is determined bytwo discrete values of the current through the pole: −1 and +1.Preferably, N is an odd number, starting from N=3, in order to enable asimple construction of the discharge lamp.

The invention can be advantageously applied in a high-pressure dischargelamp with a metal filling in the discharge vessel, such as high-pressuremercury lamps and high-pressure sodium lamps. Other suitable metals areTh, Li, Zn, Sc and In. An example of the invention relates to ametal-halide lamp. Examples of metal halides that can be used as thefilling constituent of the discharge vessel are NaI, TlI, InI, ScI₃,DyI₃, HoI₃, TmI₃, CeI₃, SnI₂, CaI₂, LiI, ThI₄ and SnCl₂ and mixturesthereof. As a result of the complex discharge cycle taking place in themetal-halide lamp, the measure in accordance with the invention canparticularly advantageously be used in the metal-halide lamp to obtain adischarge channel during lamp operation which largely coincides with thedischarge axis.

The invention will be explained with reference to a drawing.

In the drawing:

FIG. 1 is a side elevation of a discharge lamp in accordance with theinvention,

FIG. 2 is a plan view of the discharge lamp in accordance with FIG. 1,and

FIG. 3 is a plan view of a different preferred embodiment of a dischargelamp in accordance with the invention.

FIG. 1 is a side elevation of a discharge lamp 1 in accordance with theinvention. Said discharge lamp 1 comprises an outer bulb 2, which outerbulb is provided at one end with a lamp cap 3. The outer bulb 2 isprovided with a discharge vessel 4, a first pole 5 and a second pole 6located at a distance from the first pole 5. The first pole 5 and thesecond pole 6 are connected to, respectively, a first electrode 16 and asecond electrode 17. The electrodes 16, 17 are positioned in thedischarge vessel 4, and, in the operating state of the lamp, a dischargechannel (not shown) extends between said electrodes. The shortestconnection between electrodes 16, 17 is formed by the discharge axis 40.The second pole 6 is provided with different parts 7, 8, 9 which arepositioned laterally with respect to the discharge axis 40 of thedischarge vessel 4. If an electric current flows through the second pole6, the parts 7, 8, 9 generate a tangential magnetic field. As parts 7, 9and part 8 generate magnetic fields which extend partly in oppositedirections, compensation takes place at the location of the dischargevessel 4. The degree of compensation depends on the exact positioning ofparts 7, 8, 9 with respect to the discharge vessel 4. In the case of anelongated conductor it applies that the size of the magnetic field,generated by the conductor, in a point at a distance d from theconductor is inversely proportional to the distance d. FIG. 1 shows thedistance between parts 7, 9 and the discharge axis 40, which distance isreferenced d2. FIG. 1 also shows the distance between part 8 and thedischarge axis 40, which distance is referenced d1. FIG. 2 is a planview of the discharge lamp 1 in accordance with FIG. 1. The dischargevessel 4 is connected to the second pole 6. In the case shown, thedistance d2 is twice the distance d1. At such an orientation of theparts 7, 8, 9 of the second pole 6, the resultant magnetic field at thelocation of the discharge axis 40 is minimal as a result ofsubstantially complete compensation. After all, as has been describedhereinabove, superposition of magnetic fields in the discharge vessel 4can be expressed as follows:${{\sum\limits_{i = 1}^{N}\quad\frac{{ni} \cdot I}{di}} \approx 0},$with N≧2 where:

-   ni=the direction of the magnetic field generated-   N=the number of parts of the second pole which are laterally    positioned with respect to the discharge axis of the discharge    vessel,-   I=the intensity of the current flowing through the discharge channel    in the operating state, and-   di=the distance between a certain part of the second pole and the    discharge axis of the discharge vessel.

If the result of this expression is 0, this means that the presence ofmagnetic fields on the discharge axis 40 of the discharge vessel 4 isreduced to a minimum. If this expression is applied to the dischargelamp shown in FIGS. 1 and 2, the following result is obtained:$\begin{matrix}{{\left\lbrack \frac{n}{d} \right\rbrack_{deel7} + \left\lbrack \frac{n}{d} \right\rbrack_{deel8} + \left\lbrack \frac{n}{d} \right\rbrack_{deel9}} =} \\{{\left\lbrack \frac{1}{2d_{1}} \right\rbrack + \left\lbrack \frac{- 1}{d_{1}} \right\rbrack + \left\lbrack \frac{1}{2d_{1}} \right\rbrack} =} \\{{\left\lbrack \frac{1}{d_{1}} \right\rbrack - \left\lbrack \frac{1}{d_{1}} \right\rbrack} = 0}\end{matrix}$

In an ideal situation, the value of the expression is 0, which resultsin substantially complete compensation of the magnetic field in thedischarge vessel.

As a result, the discharge is not brought out of position under theinfluence of a magnetic field generated in the second pole. It should beclear that the result of said expression depends only on the ratiobetween d1 and d2, not on the actual size of d1 and/or d2.

FIG. 3 is a plan view of a different embodiment of a discharge lamp 10in accordance with the invention. Said discharge lamp 10 is composed ofcomponents similar to those used for the embodiment shown in FIGS. 1 and2. Discharge vessel 11 is connected to a first pole (not shown) and asecond pole 12. The second pole 12 is provided with three parts 13, 14,15 which are positioned substantially laterally with respect to thedischarge vessel 11. The shortest distances from the parts 13, 14, 15 tothe discharge vessel 11 are in the ratio of 6x:2x:3x, respectively. Ifthe above expression is used, the following applies: $\begin{matrix}{{\left\lbrack \frac{n}{d} \right\rbrack_{deel13} + \left\lbrack \frac{n}{d} \right\rbrack_{deel14} + \left\lbrack \frac{n}{d} \right\rbrack_{deel15}} =} \\{{\left\lbrack \frac{1}{6x} \right\rbrack + \left\lbrack \frac{- 1}{2x} \right\rbrack + \left\lbrack \frac{1}{3x} \right\rbrack} =} \\{{\left\lbrack \frac{1}{2x} \right\rbrack - \left\lbrack \frac{1}{2x} \right\rbrack} = 0}\end{matrix}$

As the result of the computation in question is 0, a minimum will lie,in the ideal case, on the discharge axis of the discharge vessel 11.

It will be clear that apart from the application of three parts of thesecond pole which are predominantly laterally positioned with respect tothe discharge vessel, it is also possible, of course, that a plurality(more than three) parts of the second pole are laterally positioned withrespect to the discharge vessel.

1. A discharge lamp comprising an outer bulb, which outer bulb isprovided with a lamp cap at one end, said outer bulb accommodating: adischarge vessel provided with electrodes, and a first pole and a secondpole at some distance from the first pole, which poles establish anelectric connection between the lamp cap and the electrodes, at least apart of the second pole being mainly laterally positioned with respectto a discharge axis, said discharge axis forming the shortest connectionbetween the electrodes, characterized in that the second pole ispositioned unilaterally with respect to the discharge vessel, saidsecond pole being shaped such that a magnetic field at the location ofthe discharge vessel is minimized.
 2. A discharge vessel as claimed inclaim 1, characterized in that the second pole is provided with a numberof successive parts which are laterally positioned with respect to thedischarge axis in the discharge vessel, which parts are spaced apart. 3.A discharge lamp as claimed in claim 2, characterized in that themagnetic fields generated by the parts of the second pole are orientedin at least two directions.
 4. A discharge lamp as claimed in claim 2,characterized in that the shortest distance between at least two partsand the discharge vessel is different.
 5. A discharge vessel as claimedin claim 4, characterized in that the parts of the second pole arepositioned in such a manner with respect to each other that thefollowing applies:${{\sum\limits_{i = 1}^{N}\quad\frac{{ni} \cdot I}{di}} \approx 0},$with (N≧0.2) where: ni=the direction of the magnetic field generated,N=the number of parts of the second pole that are laterally positionedwith respect to the discharge axis of the discharge vessel, I=theintensity of the current flowing through the discharge channel in theoperating state, and di=the shortest distance between a certain part ofthe second pole and the discharge axis of the discharge vessel.
 6. Adischarge vessel as claimed in claim 1, characterized in that thedischarge vessel is embodied so as to be hermetical-tight, the dischargevessel accommodating at least one metallic element.
 7. A discharge lampas claimed in claim 6, characterized in that the metallic element formspart of a metal halide.
 8. A discharge lamp as claimed in claim 1,characterized in that the outer bulb is provided with a diffuse layer.9. A discharge lamp as claimed in claim 1, characterized in that theouter bulb is provided with a fluorescent layer.